(1) Field
The present disclosure relates generally to spoolable tubing, and more particularly to spoolable tubing or pipes capable of operating in a pressure range that may be considered below the pressure ranges generally suited to fiber reinforced composite or metallic spoolable pipe, but above the pressure ranges generally suited to unreinforced polymeric pipes.
(2) Description of Relevant Art
Spoolable tubing, or tubing capable of being spooled upon a reel, is commonly used in numerous oil well operations, although other applications exist. For example, oil well operations include running wire line cable down hole with well tools, working over wells by delivering various chemicals down hole, and performing operations on the interior surface of the drill hole. The tubes are spoolable so that a tube can be used with one well, and then transported on a reel to another well at a different location. Steel coiled tubing is typically capable of being spooled because the steel used in the product exhibits high ductility (i.e., the ability to plastically deform). Unfortunately, the repeated spooling and use of steel coiled tubing causes fatigue damage that can cause the steel coiled tubing to fracture and fail, often without notice. The hazards of operating steel coiled tubing, i.e., risk to personnel and high economic cost resulting from down-time needed to retrieve the broken tubing sections, forces steel coiled tubing to be retired after a relatively few number of trips into a well.
Steel coiled tubing has also proven to be subject to expansion after repeated uses. Tube expansion results in reduced wall thickness with the associated reduction in the pressure carrying capability of the steel coiled tubing. Steel coiled tubing known in the art is typically limited to an internal pressure up to about 5,000 psi. Accordingly, higher pressure and continuous flexing typically reduces the steel tube's integrity and service life.
For example, the present accepted industry standard for steel coiled tube is an A-606 type 4 modified HSLA steel with yield strengths ranging from 70 ksi to 80 ksi. The HSLA steel tubing typically undergoes bending, during the deployment and retrieval of the tubing, over radii significantly less than the minimum bending radii needed for the material to remain in an elastic state. The repeated bending of steel coiled tubing into and out of plastic deformation induces irreparable damage to the steel tube body leading to low-cycle fatigue failure.
Additionally, when steel coiled tubing is exposed to high internal pressures and bending loads, the isotropic steel is subjected to high triaxial stresses imposed by the added pressure and bending loads. The high triaxial stresses result in significant plastic deformation of the tube and diametral growth of the tube body, commonly referred to as “ballooning”. When the steel coiled tube experiences ballooning, the average wall thickness of the tube is reduced, and often causes a bursting of the steel tube in the area of decreased thickness.
Steel coiled tubes also experience thinning of the tube walls due to the corrosive effect of materials used in the process of working over the well and due to materials located on the inner surface of the well bore. The thinning resulting from corrosive effects of various materials causes a decrease in the pressure and the tensile load rating of the steel coiled tubing.
Spoolable tubing can also be installed in permanent applications such as in transport of oil and gas and produced materials from wells, or injection of materials into wellbores. Typically in these applications the spoolable pipe is buried, but it can also be installed on surface. Spoolable pipe can also be installed vertically in wellbores in permanent applications including production tubing, casing, or other conduits from surface.
When the ends of a tube are subjected to opposing forces, the tube is said to be under tension. The tensile stress at any particular cross-section of the tube is defined as the ratio of the force exerted on that section by opposing forces to the cross-sectional area of the tube. The stress is called a tensile stress, meaning that each portion pulls on the other.
With further reference to a tube subjected to opposing forces, the term strain refers to the relative change in dimensions or shape of the tube that is subjected to stress. For instance, when a tube is subjected to opposing forces, a tube whose natural length is L0 will elongate to a length L1=L0+ΔL, where ΔL is the change in the length of the tube caused by opposing forces. The tensile strain of the tube is then defined as the ratio of ΔL to L0, i.e., the ratio of the increase in length to the natural length.
The stress required to produce a given strain depends on the nature of the material under stress. The ratio of stress to strain, or the stress per unit strain, is called an elastic modulus. The larger the elastic modulus, the greater the stress needed for a given strain.
For an elastomeric type material, such as used in tubes, the elongation at break may be high (typically greater than 400 percent) and the stress-strain response may be highly nonlinear. Therefore, it is common practice to define a modulus of elasticity corresponding to a specified elongation. The modulus for an elastomeric material corresponding to 200 percent elongation typically ranges form 300 psi to 2000 psi. In comparison, the modulus of elasticity for typical plastic matrix material used in a composite tube is from 100,000 psi to 500,000 psi or greater, with representative strains to failure of from 2 percent to 10 percent. This large difference in modulus of elasticity and strain to failure between rubber and plastics and thus between tubes and composite tubes may permit a tube to be easily collapsed to an essentially flat condition under relatively low external pressure. This large difference may also eliminate the spoolable pipe's capability to carry high axial tension or compression loads while the higher modulus characteristic of the plastic matrix material used in a composite tube is sufficiently stiff to transfer loads into the fibers and thus resist high external pressure and axial tension and compression without collapse.
The procedure to construct a composite tube to resist high external pressure and compressive loads involves using complex composite mechanics engineering principles to ensure that the tube has sufficient strength. Such a composite tube is presented in U.S. Pat. Nos. 5,921,285, 6,016,845, 6,148,866, 6,286,558, 6,357,485, and 6,604,550 the entireties of which are incorporated herein by reference in their entireties. There are some applications in which the high external pressures for which such composite pipes are desirable, may not be present, and thus, other types of reinforced spoolable pipe may be preferable.